Schreckstoff - Kin Selection Hypothesis

Kin Selection Hypothesis

One of the first hypotheses for the evolution of schreckstoff centered on W.D. Hamilton’s theory of kin selection (Hamilton 1963, 1964a, 1964b). Under the theory of kin selection, the sender of the chemical alarm signal would be willing to incur the costs of sending this signal if the benefits to related individuals were sufficiently high. In a situation where the sender of the signal is paying great costs (i.e., it releases the chemical alarm signal because it has incurred potentially mortal mechanical damage), the benefits to closely related kin would have to be great. Under the framework of kin selection, behaviors that are seemingly detrimental to the sender are selected for because they benefit individuals who are likely to share alleles by common descent. In this way, the frequency of the sender’s alleles in the next generation is increased via their presence in successful kin.

In order to apply kin selection theory to the evolution of schreckstoff, a number of conditions must be met. First, there must be evidence that the release of schreckstoff by the sender confers benefits to the receivers. Second, it must be shown that individuals in the order Ostariophysan associate mainly with family members. If either of these two assumptions is violated, then the kin selection hypothesis would not be supported.

Some evidence exists in support of the first assumption that the release of schreckstoff confers quantifiable advantages to the receivers of this chemical signal. A laboratory experiment conducted by Mathis and Smith (1992) revealed that fathead minnows exposed to conspecific schreckstoff survived 39.5% longer than controls when placed in a tank with a predatory northern pike (Esox lucius). This finding suggests that schreckstoff increases vigilance in receivers, resulting in a quicker reaction time following detection of the predator.

The second assumption, that individuals in the order Ostariophysan associate with close family members, does not appear to be supported by empirical evidence. Naish et al. (1993) examined shoals of European minnows (Phoxinus phoxinus) and found no difference in relatedness within and between shoals, indicating that individuals are not associating more closely with relatives than nonrelatives. Shoal composition has not been examined in all members of the Ostariophysan order and shoals composed entirely of family members may yet be discovered. Nevertheless, the finding that schreckstoff production is maintained in a species where the function is clearly unrelated to kin benefits provides strong evidence against kin selection as a mechanism for the evolution of schreckstoff.

Fathead minnows have also been found to produce fewer epidermal alarm substance cells (and therefore less schreckstoff) when in the presence of familiar shoalmates (Wisenden and Smith 1998). The results of this study indicate one of two scenarios, neither of which are compatible with the hypothesis that schreckstoff evolved via kin selection. First, if schreckstoff evolved via kin selection, we would expect more epidermal alarm substance cells to be produced in the presence of kin than nonkin. This means that familiar shoalmates in fathead minnows should be closely related kin and schreckstoff production should be increased when in shoals with familiar individuals. The Wisenden and Smith (1998) study did not find this to be the case. Second, these results indicate that individuals either do not associate with kin at all or that production of schreckstoff varies depending on how familiar the focal fish is with the individuals with which it shoals. In conclusion, evidence does not support the hypothesis that schreckstoff evolved because it bolstered the inclusive fitness of the sender through increased survival of kin.